Threespot dascyllus

The threespot dascyllus (Dascyllus trimaculatus), also called the domino damsel or just domino, belongs to the damselfish family Pomacentridae. It lives across the Indo-Pacific, from the Red Sea and East Africa out to the Pitcairn Islands, as far north as southern Japan, south to Australia, and in parts of the Philippines.

This fish is one of four species in the D. trimaculatus species complex, which also includes D. albisella, D. strasburgi, and D. auripinnis. The ranges of D. trimaculatus and D. auripinnis overlap in the Northern Cook Islands, but otherwise each of the four species occupies its own separate area. The whole complex split from the D. reticulatus complex about 3.9 million years ago, during the Pleistocene. Analysis of mitochondrial genomes shows that D. trimaculatus itself divides into three populations: one in the Indian Ocean, one in southern French Polynesia, and one in the West-Central Pacific. D. auripinnis clusters with the West-Central Pacific population of D. trimaculatus, which casts some doubt on whether it is truly a separate species. Fish in Fiji that are labeled D. trimaculatus have yellow pelvic and anal fins and orange streaks in the dorsal and caudal fins; they look intermediate between the two species, and genomic work indicates they come from repeated backcrossing that introduced D. auripinnis genes into D. trimaculatus.

The name *trimaculatus* means "three-spotted," referring to the fish's three white spots—the same feature that gives it the common name "domino." Its body is grey to black, with two white spots on the sides and one between the eyes, like a domino piece. It can reach 13 cm (5.1 in) long. The coloring varies: the forehead spot may be missing, and the side spots can be very small. It eats algae, copepods, and other planktonic crustaceans.

Adults usually gather in small groups around coral heads or large rocks. Juveniles often live with large sea anemones or hide among the spines of diadema sea urchins or branching corals. The species can be found as deep as 55 m (180 ft). Like other members of its species complex, D. trimaculatus recognizes its own kind by sound.

Juveniles take shelter with anemones such as *Stichodactyla mertensi*, but they lose this relationship as they grow up. Very rarely, juveniles may use corals instead.